A lighting apparatus or indicating apparatus, for example apparatus for interior lighting, traditionally includes an opaque base plate which is provided with devices (lamp holders) at appropriate positions, for the fitting and electrical connection of one of more filament lamps. Such an apparatus also includes a light transmitting element in the form of a cover glass or globe which is fitted in front of the base plate (considered with respect to the direction in which light is emitted). The base plate and the cover glass may, conventionally, include light-controlling means for giving the beam or beams produced by the apparatus the required photometry. Such light-controlling means may for example be in the form of reflective elements on the base plate, or lenses, prisms or ribs on the cover glass. The apparatus may be arranged to emit one or more beams, and in particular signalling or indicating beams.
However, such a conventional arrangement has a certain number of drawbacks, in particular where the apparatus consists of, or includes, indicating (or signalling) lights. This is true both from the optical point of view and from the aesthetic point of view. First of all, in the optical context, it is quite difficult, especially for indicating functions where the illuminated zone of the apparatus has a large area, to obtain a homogeneous light intensity over the whole of this area. In particular, those regions of this illuminating zone which are furthest away from the filament will in general receive a quantity of light per unit of surface area which is smaller than that received by the region which lies immediately in front of the lamp; the light intensity in the illuminated zone decreases from its centre towards its edges.
In addition, from the aesthetic point of view, the light transmitting element (i.e. the cover glass) will be made in one or more colours (e.g. red, amber and so on), the colour being determined by the colour which the emitted beam is required to have. In addition, due to the fact that each indicating function may make it necessary to provide particular light-controlling means on the cover glass in order to give the emitted beam its required shape, the cover glass, when seen as a whole, usually looks extremely unhomogeneous when all the various lighting and/or indicating functions of the apparatus are extinguished.
Again, conventional lighting or indicating apparatuses generally give only a rather mediocre light output. Besides all this, filament lamps give rise to a substantial amount of heat, which in turn leads to design problems, especially in respect of the choice and dimensioning of the materials of components which are close to these lamps.
Finally, conventional lighting or indicating apparatus is relatively bulky, due especially to the fact that it is necessary to leave around the lamp a free space of significant size, mainly because of the heat which is produced as mentioned above.
It is also known to use a discharge lamp in a motor vehicle headlamp. Discharge lamps are well known for their high light output. However, since such a lamp emits very intense radiation from a highly localised region, it is not itself suitable for overcoming the problems mentioned above.